A color filter wheel rotating in front of a monochrome charge-coupled device (CCD) sensor can be used to produce high resolution still images without the color artifacts normally associated with images produced through a color filter array mounted on a CCD sensor. Since this technique requires three sequential color records obtained by three separate exposures through the rotating color filter wheel, movement of the camera body enclosing the CCD sensor, or slight movement of the object being sensed, may cause the position of the CCD sensor relative to the object to be slightly different for one or more of the color records. In the prior art, the relative pixel shift between the records is determined, and then the pictures are either retaken until a suitable picture (without movement) is obtained, or the three misregistered records are manually aligned.
As an example of the former, U.S. Pat. No. 5,164,824 discloses an endoscope that generates color images from sequences of red, green, and blue illuminations. The endoscope also has an image freeze feature, but encounters difficulty when there is color misregistration due to movement between the red, green, or blue images. The solution is to monitor movement and to permit a freeze frame only when the movement is less than a predetermined level. In essence, therefore, actual misregistration is not corrected, but the image is retaken until the three records are relatively free of motion.
As an example of the technique of manual adjustment, several features of the Adobe Photoshop.TM. software may be used as tools to shift image planes relative to one another. The offset filter feature can be used to shift a plane by an integer number of pixels. The original color image is separated into its red, green, and blue component parts, and the proper plane is selected for shifting. The user must determine the amount of the shift via trial and error. After the shifting has been done, the three component planes are recombined to view the final result.
Fractional pixel shifts can also be obtained using the custom filter option. Again, the original color image is separated into red, green, and blue components. The user must determine the amount of the shifting to be done and calculate the filter coefficients necessary to perform that shift. The three planes are recombined to view the result. Both fractional and integer pixel shifts require user intervention. This process is manual and error-prone; most important, it does not determine the proper shift required, but simply performs the shift operation once the user decides what shift values to use.
In copending Ser. No. 712,865, filed Jun. 10, 1991 by the present assignee and entitled "Cross Correlation Image Sensor Alignment System", a correlation technique is used to register images from a 3-chip camera automatically, but it requires a special test target and does not compensate for any registration shifts from picture to picture. What is needed is a method for automatically registering images from a color sequential camera based on the picture image itself.